In seismically active locations, tilt-up concrete buildings require extra steel reinforcement in the prefabricated wall panels. Typically reinforcing bars called, chord bars, are cast into the concrete walls and run the continuous length of the wall. The chord bars ensure that the walls remain standing during laterally imposed seismic forces. Because if walls collapse the roof will also collapse endangering lives.
Conventionally the method of joining chord bars in adjacent tilt-up wall panels is to use field welding techniques. In this method the adjacent walls are erected with a small gap between adjacent panels. Chord bars cast into each wall panel have their ends exposed in a notch at the respective edges of adjacent panels. The chord bar from one panel is connected to a chord bar in an adjoining panel by a short length of steel angle that is placed behind the adjacent chord bars and welded in place.
In the structural design of the connection the size and length of the weld is calculated to develop the full strength of the reinforcing chord bars. A significant disadvantage of this field-welded connection is the lack of reliability and quality, and therefore strength of the weld. Typical steel reinforcing bars for concrete conform to standard testing and material designations and are generally not weldable grades of steel. Weldable reinforcing bars are available, but this is expensive and requires special ordering.
Also, in order to produce the quality field-weld on typical reinforcing bars, the bars need to be preheated prior to welding and cooled after welding in accordance with established requirements. The preheating and cooling procedures are very difficult to accomplish successfully in the field. If the proper procedures are not carefully followed, the weld produced will be brittle and weak. Thus there is much concern in the prefabricated building industry about the quality of welds, and how the connections will perform under stress such as seismic activity applying lateral loads to the tilt-up concrete structures. If the welds break prematurely the walls can collapse.
Administrative officials in charge of building safety recognize the importance of the field welds and require an inspector on-site during the welding process. The inspector, however, does not necessarily witness each weld as they are very often high above the ground. Hence reliability in the quality of field-welding chord bar connections is still lacking.
Another disadvantage of this type of prefabricated wall connection is that it rigidly locks one panel to another which can result in cracks in the concrete panels from environmental causes of temperature and moisture changes. Cracks weaken the wall, are asthetically unattractive and permit water to leak into buildings.
Moisture changes in concrete result in volume changes of the material. Drying of concrete from water evaporation causes a decrease in volume of the concrete. This behavior is called drying shrinkage. The prefabricated tilt-up wall panels are usually erected when the concrete is only about ten days old. Drying shrinkage can be substantial during the first month or two of the concrete from exposure to sun and wind. Restraining adjacently connected panels from moving while drying shrinkage is occuring causes tensile stresses which can produce cracks. If the panels are permitted to freely move the development of cracks can be prevented. The present method of field-welding chord bar connections prevents panel movement; hence cracks can occur.
Seasonal variations in temperature can cause expansion and contraction movements in concrete panels that exceed the dry shrinkage movement. Thus, the expansion and contraction due to seasonal changes can also cause cracks in rigidly restrained panels.
It is one object of the present invention to provide a chord bar connecting system for joining adjacent prefabricated wall panels which provides superior structure reliability.
Yet another object of the present invention is to provide a chord bar connecting system for adjoining adjacent prefabricated concrete walls, which allows free movement of joined wall panels to minimize or prevent cracks.
Still another object of the present invention is to provide a chord bar connecting system in which adjacent prefabricated wall panels can be immediately secured after erection.
Yet another object of the present invention is to provide a chord bar connection system which compensates for misalignment between adjacent erected panels. Misalignment compensation is provided for by utilizing oversized holes in adjacent connecting face plates of the chord bar connectors.
Yet another object of the present invention is to provide a chord bar connector system which may be shop-welded to provide quality welds not available with field-welded techniques.
Yet another object of the present invention is to provide a chord bar connector system in which crushable spacers can be utilized to compensate for wall movement after erection.
Yet another object of the present invention is to provide a chord bar connector system utilizing face plates welded to chord bars and then adjacent face plates joined with bolts. Shims looseley fill the gap space between adjacent face plates.
Yet another object of the present invention is to provide chord bar connector for joining adjacent prefabricated concrete panels which substantially reduces concrete cracks due to expansion, contraction warping, or curling of walls.
Yet another object of the present invention is to provide a bolted chord bar connecting system which improves safety during construction should forces due to seismic activity occur. Adjacent prefabricated walls can be connected immediately after erection to provide improved safety against collapse.
Yet another object of the present invention is to provide a chord bar connecting system for joining adjacent wall panels which reduces construction schedule time.
The above and other objects, advantages and novel features of the invention will be more fully understood from the following detailed description and the accompanying drawing in which: